From the viewpoint of reliability, seamless steel pipes are widely used mainly in applications such as oil country tubular goods (OCTG), line pipes, and the like that are required to have high corrosion resistance and toughness. Seamless steel pipes made of various kinds of low-alloy steels are used in these applications. In manufacturing the seamless steel pipes, in order to increase the strength properties and toughness, the steel pipes are often subjected to heat treatment of hot rolled pipes such as quenching and tempering. As a method for heat treatment such as quenching and tempering, a conventional reheating and quenching process has been practiced, wherein the hot rolled pipes are once cooled and then reheated to the Ac3 transformation point or a higher temperature in an offline heat treatment furnace followed by quenching, and thereafter tempered at a temperature not higher than the Ac1 transformation point. At the same time, however, from the viewpoint of saving process steps and energy, a direct quenching process has been investigated and improved, wherein the as-rolled hot pipes are immediately direct quenched from the Ar3 transformation point or a higher temperature that is based on a potential heat of the as-rolled hot pipes, and then tempered.
Patent Document 1 has disclosed a method for manufacturing high-strength steel pipes excellent in sulfide stress-corrosion cracking resistance, comprising steps of working continuously cast billets of a low-alloy steel having a specific composition into seamless steel pipes at a temperature not lower than the Ac3 transformation point, direct quenching the steel pipes, reheating the steel pipes to the temperature range from the Ac3 transformation point to a temperature of the Ac3 transformation point+100° C., and quenching the steel pipes again from this temperature, and a step of tempering the steel pipes at a temperature not higher than the Ac1 transformation point. This is a method in which reheating and quenching are added before the tempering step of simple direct quenching process. With this method, the sulfide stress-corrosion cracking resistance is improved significantly by a grain refinement as compared with the simple direct quenching process.
Patent Document 2 has, similarly to Patent Document 1, disclosed a method for manufacturing high-strength steel pipes that comprises a step of performing reheating and quenching after direct quenching, wherein the steel pipes are direct quenched and tempered under specific conditions to control precipitated carbides.
Patent Document 3 has disclosed a method for manufacturing high-strength seamless steel pipes excellent in sulfide stress cracking resistance (hereinafter, referred to as “SSC resistance”) in which billets of a low-alloy steel having a specific composition are hot pierced and hot rolled to produce seamless steel pipes. In this method, the billets are pierced and then finish rolled at a reduction of area of 40% or more at the finishing temperature of 800 to 1050° C., thereafter being subjected to “reheating” under specific conditions in the temperature range of 850 to 1100° C., and then the steel pipes are immediately subjected to “direct quenching”, and are tempered at a temperature not higher than the Ac1 transformation point. This Document also describes a method in which reheating and quenching are performed once or twice after the “direct quenching.”
The term “reheating” described in claim 1 of Patent Document 3 refers to not a reheating from the normal temperature, but refers to a reheating performed on the way from the finish rolling step to the direct quenching step, and therefore corresponds to the “complementary heating” in this description. Patent Document 3 describes that this “reheating” contributes to making crystal grains fine as recrystallizing treatment. The term “direct quenching” is used in Patent document 3, and the process of the “direct quenching” and the precedent process correspond to the inline heat treatment in this description. That is, Patent Document 3 relates to a technique of improved inline heat treatment method, or a technique in which the reheating and quenching are combined with inline heat treatment step.
Patent Document 4 also has disclosed a method for manufacturing seamless steel pipes. In this method, after piercing-rolling has been performed at a specific strain rate, the pipes are rolled at a specific average strain rate, at a working ratio of 40% or more, and at a finishing temperature of 800 to 1050° C. using a rolling mill group in which a continuous elongation rolling mill and a finish rolling mill are arranged closely. Thereafter, the produced steel pipes are quenched to a temperature not higher than the Ar3 transformation point at a cooling rate of 80° C./minute or higher, the cooled steel pipes are reheated to 850 to 1000° C., and then are subjected to a process of quenching and tempering in succession.
This method for manufacturing seamless steel, in which the steps are carried out on a series of continuous lines, is characterized in that after the completion of finish rolling at a high temperature, the steel pipes are cooled to a temperature not higher than the Ar3 transformation point (the cooling is stopped halfway), and thereafter are reheated, whereby reverse transformation from ferritic phase of body-centered cubic structure (BCC) to austenitic phase of face-centered cubic structure (FCC) is allowed to take place.